1. Field of the Invention
The present invention relates to a suspension system for a vehicle which has a hydraulic cylinder provided between a vehicle body and an axle. More particularly, the present invention relates to a suspension system which is designed so that when the vehicle is running on a normal road surface, the suspension system absorbs any changes in the road surface, thereby stably maintaining the attitude of the vehicle body, and in the case of large irregularities, it is possible to prevent full rebound or full bump which would otherwise occur when the hydraulic cylinder performs a full-stroke operation.
2. Description of the Prior Art
There has heretofore been a conventional suspension system for a vehicle, known as an active control type suspension system, wherein a hydraulic cylinder is provided between a vehicle body and an axle, and the level of pressure generated in the hydraulic cylinder is controlled in accordance with a value obtained by multiplying a detected vertical speed of the vehicle body by a negative constant, thereby preventing vibration of the vehicle body.
The conventional suspension system is arranged to calculate a vertical speed of a portion of the vehicle body at the position of each wheel by integrating the output value from an acceleration sensor provided for each wheel. For this reason, the prior art suffers from the problem that much time is needed for the integral calculation and hence the response is slow and that, since errors in the acceleration sensors are enlarged by the integration, a slight error in the acceleration sensors makes it impossible to obtain satisfactory vibration-damping characteristics.
The present applicant has proposed a suspension system (Japanese Patent Application No. 01-100590 (1989)) which is free from the above-described problems.
More specifically, the proposed suspension system comprises a hydraulic cylinder that extends and retracts in accordance with a change in the distance between a vehicle body and an axle to vary the volumetric capacity of a working chamber defined therein, an accumulator connected to the working chamber in the hydraulic cylinder and containing a gas and an oil which is capable of flowing in and out of the accumulator, a flow control valve controlling the flow rate of oil charged into and discharged from the accumulator and the hydraulic cylinder, a vehicle level sensor detecting the distance between the vehicle body and the axle, an acceleration sensor detecting the vertical acceleration of the vehicle body, and a control unit controlling the flow control valve on the basis of a first flow control signal obtained by multiplying the detected signal from the vehicle level sensor by a first amplification factor and a second flow control signal obtained by multiplying the detected signal from the acceleration sensor by a second amplification factor.
The above-described suspension system has excellent vibration-damping characteristics and also exhibits high response because it does not involve the execution of an integral calculation such as that described above. However, since the first amplification factor must be set to a small value in order to absorb not only steep changes of the road surface but also gradual changes thereof to thereby stabilize the attitude of the vehicle body, even if the hydraulic cylinder is near the end of its stroke, the controlled variable is small, whereby full rebound or full bump is likely to occur.
It should be noted here that "full rebound" describes a state where a vehicle body rebounds in response to an impact to which it is subjected when the hydraulic cylinder extends as far as the end of its extension stroke, while "full bump" refers to a piston impacting against the cylinder end when the hydraulic cylinder retracts as far as the end of its retraction stroke.